Pressure Prediction in the Shallow Ursa Basin: Deepwater Gulf of Mexico

Overpressures measured with pore pressure penetrometers during Integrated Ocean Drilling Program (IODP) Expedition 308 reach 70 % and 60% of the hydrostatic effective stress (lambda* =0.7 and 0.6) in the first 200 meters below sea floor (mbsf) at Sites U1322 and U1324, respectively, in the deepwater Gulf of Mexico, offshore Louisiana. We conducted extensive uniaxial consolidation tests on whole core samples to obtain the consolidation properties of the Ursa mudstones. The results suggest that the compression index linearly decreases with in situ void ratio. We show that the relationship of compressibility index versus void ratio can be obtained from a single consolidation test by compressing the soil over a large range in effective stress. A virgin compression curve can then be constructed based on this relationship to predict pore fluid pressure. In the Ursa Basin, this new approach successfully predicted pressures interpreted from the penetrometer measurements within the non-deformed sediments. We interpret that the high overpressures observed are driven by rapid sedimentation of low permeability material from the ancestral Mississippi River.